One of conventional ultrasonic flowmeters of the type described above is disclosed, for example, in Japanese Laid-Open Publication No. 8-233628. In this ultrasonic flowmeter, as shown in FIGS. 27A and 27B, ultrasonic oscillators 2 and 3 are disposed facing each other in a portion of a flow path 1 which is a rectangle in section 4. The speed of a fluid is calculated by a flow calculation means 5 using the difference between the propagation time in which an ultrasonic wave transmitted from the ultrasonic oscillator 2 is received by the ultrasonic oscillator 3 and the propagation time in which an ultrasonic wave transmitted from the ultrasonic oscillator 3 is received by the ultrasonic oscillator 2. At the same time, the flow rate distribution in the flow path 1 is presumed from the Reynolds number of the fluid, to obtain a correction coefficient and thus calculate the flow.
The conventional ultrasonic flowmeter, however, has the following problem. The propagation distance of a reflected wave which is reflected from the inside walls of the flow path is different from the propagation distance of a direct wave which propagates without being reflected. A phase difference is therefore generated between the reflected wave and the direct wave. Since a synthetic wave of the reflected wave and the direct wave is observed as a received wave, the amplitude of the received wave increases or decreases, and the period thereof varies, depending on the phase difference between the reflected wave and the direct wave. This narrows the measurement precision as well as the measurable flow range.